This invention relates to thermal transfer printing apparatus and in particular to obtaining optimum print quality with such apparatus.
Known thermal transfer printing apparatus comprises a print head having a plurality of selectively energisable thermal printing elements disposed in a line. A print receiving medium is fed in a direction transverse to the line of thermal printing elements. A thermal transfer ink ribbon is fed with the print receiving medium and interposed between the print receiving medium and the printing elements. The ink transfer ribbon carries a layer of ink and the ink ribbon is orientated such that the layer of ink is in contact with the print receiving medium. An impression roller is disposed in opposition to the line of printing elements and is resiliently biased toward the printing elements so as to bring the print receiving medium into intimate ink transfer engagement with the ink layer of the ribbon and to bring the ribbon into heat transfer engagement with the printing elements. During feeding of the print receiving medium and the ink ribbon, the thermal printing elements are selectively energised in a series of printing cycles to heat selected areas of the ink layer. The heating of the selected areas of the ink layer results in those selected areas of ink adhering to the print receiving medium and after passing the line of thermal printing elements the used ink ribbon is peeled from the print receiving medium to leave a required imprint consisting of the selected areas of ink on the print receiving medium.
Thermal transfer printing apparatus is used in postage meters to print postage indicia on mail items, the postage indicia imprint providing evidence that accounting for postage charges in respect of the mail items has been effected. Mail items may have a thickness within a relatively large range of thicknesses and hence, unlike printing apparatus for printing on sheets of paper having a thickness within a relatively restricted range of thicknesses, printing apparatus used in postage meters is required to be capable of printing on mail items having relatively large range of thicknesses. In printing apparatus for printing on sheets within a relatively restricted range of thicknesses the elements of the printing apparatus may be mounted and arranged to operate in a fixed unvarying relationship. However in printing apparatus for use in postage meters required to accommodate a relatively large range of thicknesses of mail items it has been found that optimum quality is not obtained in respect of mail items of differentthicknesses.